Facilitatory and inhibitory effects of xanthines and NaF were studied at cholinergic synapses. It was shown that xanthines exhibit a differential facilitatory effect at the neuromyal junction but not at the ganglion. The facilitation was both pre and post-synaptic in nature. Postsynaptically, xanthines and NaF appeared to stabilize the ionic channels in open position. The facilitation occurred also following inhibition of cholinesterase; thus, these compounds may be useful as adjuncts in the treatment of myasthenia gravis patients which is consistent with our past findings of their usefulness in this disease. Evidence was obtained that morphological, electro-physiological and pharmacological change occurs during myasthenic process, with a shift in the frequency of the occurrence of type I ,II and III fibers. At the ganglia, xanthines cause depolarization of the postsynaptic membrane and augmentation of the inhibitory potential. The dopaminergic nature of the inhibitory potential was confirmed; it was shown that cAMP does not participate in the generation of the potential. Finally, participation of central cholinergic system in pain mechanism was demonstrated and investigated from the viewpoint of its possible relation to morphine receptors and serotonin mechanisms. BIBLIOGRAPHIC REFERENCES: Dun, N.J. and Karczmar, A.G. (1977). A $ comparison of the effect of theophylline and adenosine 3':5' cyclic monophosphate on the superior cervical ganglion of the rabbit by means of the sucrose-gap method. J. Pharmacol. Exper. Therap. in press. Dun, N.J. and Karczmar, A.G. (1977). The presynaptic site of action of norepinephrine in the superior cervical ganglion of the guinea pig. J. Pharmacol. Exper. Therap. in press.